The mammalian immune system has evolved a variety of mechanisms to protect the host from cancerous cells. An important component of this response is mediated by cells referred to as T cells. Cytotoxic T lymphocytes (CTL) are specialized T cells that primarily function by recognizing and killing cancerous cells or infected cells, but they can also function by secreting soluble molecules referred to as cytokines that can mediate a variety of effects on the immune system. T helper cells primarily function by recognizing antigen on specialized antigen presenting cells, and in turn secreting cytokines that activate B cells, T cells, and macrophages. A variety of evidence suggests that immunotherapy designed to stimulate a tumor-specific CTL response would be effective in controlling cancer. For example, it has been shown that human CTL recognize sarcomas (Slovin et al., 1986), renal cell carcinomas (Schendel et al., 1993), colorectal carcinomas (Jacob et al., 1997), ovarian carcinomas (Peoples et al., 1993), pancreatic carcinomas (Peiper et al., 1997), squamous tumors of the head and neck (Yasumura et al., 1993), and squamous carcinomas of the lung (Slingluff et al., 1994; Yoshino et al., 1994). The largest number of reports of human tumor-reactive CTLs, however, has concerned melanomas (Boon et al., 1994). The ability of tumor-specific CTL to mediate tumor regression, in both human (Parmiani et al., 2002; Weber, 2002) and animal models, suggests that methods directed at increasing CTL activity would likely have a beneficial effect with respect to tumor treatment.
Clinical trials using adoptive cellular therapy and active vaccination have demonstrated the importance of CD8 T-cells in controlling cancer (Morgan et al., 2006; Rosenberg, 2008; Hiugano et al., 2009; Schwartzentruber et al., 2011). A large number of tumor-associated antigens (TAA) recognized by CD8 T-cells have been identified in the last 20 years, and clinical tumor regressions have been associated with immunotherapies based on some of them (Slingluff et al., 2004; Rosenberg, 2008). However, cancer vaccines targeting a range of TAA have induced disappointing clinical response rates of 3-6% (Rosenberg et al., 2004). The repertoire of TAA include: i) neoantigens formed by mutations in cellular proteins; ii) antigens induced by oncogenic viruses; iii) cancer-testis antigens normally expressed only in germ-line cells; and iv) tissue-specific differentiation antigens (Williamson et al., 2006). Only a small number of TAA source proteins have been linked to either initial cellular transformation processes or later tumorigenic processes such as angiogenesis and metastasis (Hogan et al., 1998; Simpson et al., 2005). Targeting TAA derived from proteins that are vital for a cancer cell's survival and metastatic potential is attractive, since down-regulation and/or mutation of genes encoding these proteins as a means of immune evasion could compromise cellular malignancy (Dunn et al., 2004; Hirohashi et al., 2009).
As such, TCRs can be employed for various purposes for which how antibody molecules have been utilized. One challenge with respect to TCRs as opposed to antibodies, however, is that the former are not secreted from the cells in which they are made. This can limit the utility of TCRs as therapeutic and/or diagnostic agents. These challenges have been met to varying degrees of success by the production of soluble TCRs.
Several methods for producing soluble TCRs and TCR-like molecules have recent been reported. For example, U.S. Patent Application Publication No. 2008/0015139 of Lichterfeld et al. describes the production and use of soluble TCRs for the detection and treatment of viral infections. PCT International Patent Application Publication No. WO 2013/057586 of Walseng et al. describes various additional methods for producing soluble TCRs, such as isolation of α and β chains from bacterial inclusion bodies (see also Richman & Kranz, 2007) and STAR™ technology (Altor Bioscience Corporation, Miramar, Fla., United States of America), in which hybrid soluble TcR-Ig molecules are connected via a flexible linker (see also Mosquera et al., 2005).
Thus, soluble TCRs are useful as diagnostic and/or therapeutic tools. They can be employed to detect cells that express TAAs such as, but not limited to peptides derived from TAAs complexed with MHC molecules. Additionally, soluble TCRs can be used to deliver a therapeutic agent, including but not limited to a cytotoxic compound or an immunostimulating compound, to cells presenting a particular TAA-derived peptide.
The interaction of a TCR with HLA-bound antigens including, but not limited to a peptide derived from a TAA, results in cytotoxic T lymphocytes (CTLs) killing cells that express the antigen (e.g., a cancer cell) and/or secreting cytokines in response to a cancer cell. This process involves the interaction of the T cell receptor, located on the surface of the CTL, with what is generically referred to as an MHC-peptide complex which is located on the surface of the cancerous cell. Major histocompatibility complex (MHC)-encoded molecules have been subdivided into two types, and are referred to as class I and class II MHC-encoded molecules. In the human immune system, MHC molecules are referred to as human leukocyte antigens (HLA). Within the MHC complex, located on chromosome six, are three different loci that encode for class I MHC molecules. MHC molecules encoded at these loci are referred to as HLA-A, HLA-B, and HLA-C. The genes that can be encoded at each of these loci are extremely polymorphic, and thus, different individuals within the population express different class I MHC molecules on the surface of their cells. HLA-A1, HLA-A2, HLA-A3, HLA-B7, HLA-B14, HLA-B27, and HLA-B44 are examples of different class I MHC molecules that can be expressed from these loci.
The peptides which associate with the MHC molecules can either be derived from proteins made within the cell, in which case they typically associate with class I MHC molecules (Rock & Goldberg, 1999); or they can be derived from proteins which are acquired from outside of the cell, in which case they typically associate with class II MHC molecules (Watts, 1997). The peptides that evoke a cancer-specific CTL response most typically associate with class I MHC molecules. The peptides themselves are typically nine amino acids in length, but can vary from a minimum length of eight amino acids to a maximum of fourteen amino acids in length. Tumor antigens may also bind to class II MHC molecules on antigen presenting cells and provoke a T helper cell response. The peptides that bind to class II MHC molecules are generally twelve to nineteen amino acids in length, but can be as short as ten amino acids and as long as thirty amino acids.
The process by which intact proteins are degraded into peptides is referred to as antigen processing. Two major pathways of antigen processing occur within cells (Rock & Goldberg, 1999). One pathway, which is largely restricted to professional antigen presenting cells such as dendritic cells, macrophages, and B cells, degrades proteins that are typically phagocytosed or endocytosed into the cell. Peptides derived from this pathway can be presented on either class I or to class II MHC molecules. A second pathway of antigen processing is present in essentially all cells of the body. This second pathway primarily degrades proteins that are made within the cells, and the peptides derived from this pathway primarily bind to class I MHC molecules. Antigen processing by this latter pathway involves polypeptide synthesis and proteolysis in the cytoplasm, followed by transport of peptides to the plasma membrane for presentation. These peptides, initially being transported into the endoplasmic reticulum of the cell, become associated with newly synthesized class I MHC molecules and the resulting complexes are then transported to the cell surface. Peptides derived from membrane and secreted proteins have also been identified. In some cases these peptides correspond to the signal sequence of the proteins which is cleaved from the protein by the signal peptidase. In other cases, it is thought that some fraction of the membrane and secreted proteins are transported from the endoplasmic reticulum into the cytoplasm where processing subsequently occurs. Once bound to the class I MHC molecule, the peptides are recognized by antigen-specific receptors on CTL. Several methods have been developed to identify the peptides recognized by CTL, each method of which relies on the ability of a CTL to recognize and kill only those cells expressing the appropriate class I MHC molecule with the peptide bound to it. Mere expression of the class I MHC molecule is insufficient to trigger the CTL to kill the target cell if the antigenic peptide is not bound to the class I MHC molecule. Such peptides can be derived from a non-self source, such as a pathogen (for example, following the infection of a cell by a bacterium or a virus) or from a self-derived protein within a cell, such as a cancerous cell. The tumor antigens from which the peptides are derived can broadly be categorized as differentiation antigens, cancer/testis antigens, mutated gene products, widely expressed proteins, viral antigens and most recently, phosphopeptides derived from dysregulated signal transduction pathways. (Zarling et al., 2006).
Immunization with cancer-derived, class I or class II MHC-encoded molecule associated peptides, or with a precursor polypeptide or protein that contains the peptide, or with a gene that encodes a polypeptide or protein containing the peptide, are forms of immunotherapy that can be employed in the treatment of colorectal cancer. Identification of the immunogens is a necessary first step in the formulation of the appropriate immunotherapeutic agent or agents. Although a large number of tumor-associated peptide antigens recognized by tumor reactive CTL have been identified, there are few examples of antigens that are derived from proteins that are selectively expressed on a broad array of tumors, as well as associated with cellular proliferation and/or transformation.
Attractive candidates for this type of antigen are peptides derived from proteins that are differentially phosphorylated on serine (Ser), threonine (Thr), and/or tyrosine (Tyr; Zarling et al., 2000). Due to the increased and dysregulated phosphorylation of cellular proteins in transformed cells as compared to normal cells, tumors are likely to present a unique subset of phosphorylated peptides on the cell surface that are available for recognition by cytotoxic T-lymphocytes (CTL). Presently, there is no way to predict which protein phosphorylation sites in a cell will be unique to tumors, survive the antigen processing pathway, and be presented to the immune system in the context of 8-14 residue phosphopeptides bound to class I MHC molecules. However, thirty-six phosphopeptides were disclosed as presented in association with HLA-A*0201 on cancer cells (see Table 1 of Zarling et al., 2006).